Note: Descriptions are shown in the official language in which they were submitted.
CA 02526822 2005-11-14
Attorney Docket No.: 2802-260-017
METHOD OF SEALING THREADED CONNECTIONS
CROSS-REFERENCE TO RELATED CASE
The present application claims the benefit of the filing date of U.S.
Provisional
Application Serial No. 60/630,439, November 23, 2004, the disclosure of which
is
expressly incorporated herein by reference.
BACKGROUND OF THE INVElVTTION
The present invention relates broadly to a method of sealing connections
between
threaded pipes and other tubular sections, and more particularly to such a
method which
includes a precision calculation of the void volume for the seal ring
retaining grooves
provided in one or both of the sections for the sealing of the connection.
The oil and gas industry has devised various ways to extract crude oil and
other
subsurface hydrocarbons to support the world's energy needs. One method uses a
process called steam injection wherein a series of carefully spaced wells are
drilled into a
producing formation. Same of these wells, called "injectors," are used to
inject high
pressure steam into the formation. The hot, pressurized steam force the
hydrocarbons to
the other wells, called "producers," for recovery. Steam injection is most
often employed
in fields which are commercially marginal and which are particularly sensitive
to
recovery costs. While there are a number of casing and tubing connections in
use, many
require expensive specialty threads andlor connections with metal-to-metal
seals.
Recently, oiI and gas exploration has expanded into new frontiers, such as
deepwater drilling in water depths greater than 5,000 feet (1500 m). Wells
targeting oil
and gas reservoirs found deep below the sea floor are called HPHT (high-
pressure, high
temperature) wells. These wells offer new challenges and many operators desire
tubular
connections with multiple sealing mechanisms.
As shown, for example, in Supplementary Requirement (SR) 13_ of American
Petroleum Institute (API) Specification 5CTlISO 11960, "Specification for
Casing and
Tubing, Petroleum and Natural Gas Industries - Steel Pipes for Use as Casing
or Tubing
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for Wells," 8th Edition, 2005, seal ring grooves, i.e., "glands," are machined
into one of
the sections of the connection among the threads thereof. Seal rings,
typically formed of
a polytetrafluoroethylene or other fluoropolymer resin which may be filled or
unfilled,
are mounted in the grooves are compressed as the assembly (makeup) of the
threaded
S connection proceeds. Connections of the type herein involved and the like,
as well as
seals and gaskets therefor, are shown, for example, in U.S. Patent Nos.
6,857,668;
6,695,357; 6,669,205; 6,550,822; 5,836,589; 5,823,540; 5,689,871; 5,556,139;
5,263,748;
4,878.285; 4,753,444; 4,708,038; 4,174,846; and 4,127,927.
However, it is known that a seal ring in an unde~ll condition may have no
effective sealing ability. Conversely, a seal ring in an excess overfill
condition may
develop high, localized stresses that affect the ability of the connection to
resist applied
loading while in service.
Complicating the matter is that resins such as pol.ytetrafluoroethylene may
undergo significant volumetric expansion with increases in temperature.
Temperatuze
induced volumetric expansion of the seal ring within a "fixed" void can cause
significant,
localized stresses in the tubing or casing members, i.e., the tubing or
casing, typically
called the "pin" in the vernacular, and the coupling, typically called the
"box" or "collar"
in the vernacular, of the connection.
Moreover, steel, the typical material of construction of the connection
members
exhibits a property known as "yield stress" wherein steel stressed to a level
below its
yield stress remains elastic. In such state, stress-induced deformations in
the material
generally may be recoverable upon the removal of the load. However, when
stressed
above the yield strength, the material suffers permanent and unrecoverable
deformation.
In designing structural steel members it therefore is desirable to maintain
stress levels
below the material yield strength under all applied load conditions. Stress
levels above
yield may result in excess deformation and potential failure of the structural
members.
Assembling the connection imparts stresses into the pin and collar members of
the
connection and, in so doing, contributes to the stresses which must be
resisted by the
yield strength of the material. Such makeup stresses thereby reduce the
overall service
loading capacity of the connection. Similarly, if the seal ring is in an
excess overfill
condition or upon volumetric expansion contributes additional, localized
stresses in the
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connection as a result of the inherent incompressibility of the seal ring
material, the
ability of the connection to resist externally-applied mechanical loads can be
compromised. Failure modes can be catastrophic and may result in harm to
personnel as
well as significant damage to the wellbore, casing, tubing, drill rig and
other equipment.
Conversely, a seal ring in an unde~ll condition may have no effective
sealability and
therefore may pose many of the same risks.
In view of the foregoing, it is believed that improvements in methods for the
sealing connections between threaded pipes and other tubular sections would be
well-
received by the oil and gas industry as well as other industries so concerned.
BROAD STATEMENT OF THE INVENTION
The present invention is directed to a method of sealing connections between
threaded pipes and other tubular sections, and more particularly to such a
method which
includes a precision calculation of the void volume for the seal ring
retaining grooves
provided in one or both of the sections for the sealing of the connection. The
connection
may be between, for example, an end of an externally-threaded pipe, shaft,
casing, or
tubing, or other tubular member, i.e., "pin," and a mating, internally-
threaded coupling,
i.e., "box" or "collar," which may be used for joining the tubular member to
another such
member.
Research has shown that over-compressed seal rings impart significant stresses
in
threaded tubular connections. Additionally, volumetric expansion of seal rings
at high
temperatures, while enhancing sealability in the connection if controlled to
within
specified limits, can work to "jack" the pin and box members apart if
uncontrolled.
Indeed, if the internal seal ring grooves formed in a location of the box
having a
relatively thin cross-section, then internal pressure blocked from relief by
the seal ring
can cause the box to expand enough to release the rriatingly-threaded pin
which, in turn,
can lead to a parting failure in the connection.
This invention herein involved therefor comprehends a design methodology
which may be used in conjunction with certain seal ring materials to reduce
the potential
for such failures in service. Advantageously, such methodology affords a
designer a
greater ability to consider such aspects of the coupling design as optimum
groove size
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and location and optimum seal ring size and materials, which may be
application-
spec~c, in eliminating seal ring underfill and in limiting the amount of
overfill to within
acceptable limits.
In particular, the method of present invention allows for the precise
calculation of
S the void space defined between the groove in the coupling and mating threads
on the pin.
Such calculation may take into account the relative deformation of the pin and
box which
occurs upon the assembly of the connection. With such a precise void volume
calculation, the seal ring dimensions can be optimized over a range of
tolerance
combinations relative to the void volume in the groave after assembly. For
example,
with the knowledge imparted by the methodology of the present invention, a
designer is
provided the freedom to locate the seal ring grooves in structurally more
robust areas of
the connection and otherwise to modify the connection. As a result of such
modifications, standard connections may be made more suitable for use in
higher
pressure and temperature service where such connections otherwise would be
unsuitable
due to the effects of volumetric expansion of the seal ring at high
temperatures such as
extrusion or localized high stresses. These and other advantages of the method
herein
involved will be readily apparent to those skilled in the art based upon the
disclosure
contained herein.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and objects of the invention,
reference
should be had to the following detailed description taken in connection with
the
accompanying drawings wherein:
Fig. 1A is an axial, partially-cross-sectional, fragmentary view of the
2S caniponentry of a representative coupling connection according to the
present invention,
the coupling including a box having internally-threaded ends, and a pin having
externally-threaded ends, at least one which is threadably engageable with one
of the
ends of the box;
Fig. 1B is an axial, partially-cross-sectional, fragmentary, assembly or
"makeup"
view of the coupling of Fig, 1A including a pair of seal rings each received
in a
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corresponding internal groove of the box, the view showing one of the ends of
the pins
being thxeadably engaged with one of the ends of the box;
Fig. 2A is an axial cross-sectional, fragmentary, somewhat schematic makeup
view of showing the details of the engagement of Fig. IB in a hand-tight
condition;
Fig. 2B is a makeup view as in Fig. 2A showing the details of the engagement
of
Fig. 1B in the minimum power-tight condition, with the maximum power tight
condition
being depicted in phantom;
Fig. 3A is a 2-dimensional, axial cross-sectional, axisymmetric computer model
illustrating the deformed geometry of a simulated makeup of a coupling
connection as in
IO Figs. lA-B and 2A-B;
Fig. 3B is a model as in Fig. 3A illustrating the deformed geometry of a
simulated
makeup (assembly) to power-tight as magnified and superimposed over the
original
geometry of the coupling connection;
Fig. 4 is a model as in Figs. 3A-B showing superimposed, magnified 2-
dimensional views of the void space defined between the seal ring groove of
the box and
the mating pin thread before and after assembly;
Fig. 5 is an illustration showing the progression in the deformed geometry of
a
void volume as a helical thread propagates relative to a cylindrical groove to
define the
void volume;
Fig. 6 is a 3-dimensional solid model of the deformed geometry of the seal
ring
groove and pin threads as in Fig. 4, with the geometries of the groove and
threads being
offset to aid in the visualization thereof;
Fig. 7 is a 3-dimensional solid model showing the predicted deformed geometry
of the void volume which results from subtracting the 3-dimensional modeled
volumes of
the groove and threads of Fig: 6;
Figs. 8-11 are 3-dimensional sections taken 90° intervals of the model
of Fig. 7,
with the ends of the sections being labeled as one of A-D corresponding to the
axial
cross-sectional geometry thereat the lines A-D referenced in Fig. 7;
Fig. 12 is a plan view of a representative seal ring as in Fig. 1B; and
Fig. 13 is an axial cross-sectional view of the seal ring of Fig. 12 taken
through
line 13-I3 of Fig. 12.
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The drawings will be described further in connection with the following
Detailed
Description of the Invention.
DETAILED DESCRIPTION OF THE INVENTION
Certain terminology may be employed in the description to follow for
convenience rather than for any limiting purpose. For example, the terms
"forward,"
"rearward," "right," "left," "upper," and "lower" designate directions in the
drawings to
which reference is made, with the terms "inward," "interior," "inner," or
"inboard" and
"outward>" "exterior," "outer," or "outboard" referring, respectively, to
directions toward
and away from the center of the referenced element, and the terms "radial" or
"horizontal" and "axial" or "vertical" referring, respectively, to directions,
axes, planes
perpendicular and parallel to the central longitudinal axis of the referenced
element, and
the terms "downstream" and "upstream" referring, respectively, to directions
in and
opposite that of fluid flow. Terminology of similar import other than the
words
I5 specifically mentioned above likewise is to be considered as being used for
purposes of
convenience rather than in any limiting sense.
In the figures, elements having an alphanumeric designation may be referenced
herein collectively or in the alternative, as will be apparent from context,
by the numeric
portion of the designation only. Further, the constituent parts of various
elements in the
figures may be designated with separate reference numerals which shall he
understood to
refer to that constituent part of the element and not the element as a whole.
General
references, along with references to spaces, surfaces, dimensions, and
extents, may be
designated with arrows.
For the purposes of the discourse to follow, the precepts of the sealing
methodology of the invention herein involved are described in connection with
a standard
pin and box coupling connection used widely in the oil and gas industry. It
will be
appreciated, however, that the present invention will find applicability to
other threaded
tubular connections used in the oil and gas industry, such as integral
connections wherein
each casing or tubing member has a male end and a female end for coupling to a
corresponding end of an adjoining member, as well as to connections used in
other
industries requiring enhanced sealability for liquids or gases. The use
thereof in
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conjunction with such otter connections therefore should be considered to be
expressly
within the scope of the present invention.
Referring then to the figures wherein corresponding reference numbers are used
to designate corresponding elements throughout the several views with
equivalent
elements being referenced with prime or sequential alphanumeric designations,
a
representative threaded tubular connection is referenced generally at 10 in
the uncoupled
view of Fig. IA and in the coupled view of Fig. 1B. Such connection IO
includes a
length or other section of a pipe, casing, tube, riser, or other tubular
member or "pin," 12,
and an associated section of a tubular coupling member or "collar," 14,
configured for a
coaxial, threaded connection with the pin 12 such as along the common central
longitudinal axis thereof referenced at 16.
Pin 12 has male ends, 18a and 18b, which may be generally tapered and
externally .threaded, such as at 20a and 20b. Collar I4, in turn, has female
ends, 22a and
22b, each of which may be tapered and internally threaded, such as at 24a and
24b, to be
I5 threadably matingly engageable with a corresponding end 18a or 18b of pin
12 received
coaxially therein. In this regard, the inner diameter of the box ends 22a-b is
sized to be
threadably receivable within the outer diameter of the corresponding pin end
18a or 18b.
The threadform of the pin threads 20a-b and the mating collar threads 24a-b
may
be API buttress threads or other form which is generally square in cross-
section, but
alternatively may be API "standard" threads other generally triangular threads
or other
form. Threadforrns of the type herein involved are further described in API
Standard RP
5B, "Threading, Gauging, and Thread Inspection of Casing, Tubing, and Line
Pipe
Threads," 14~' ed., August, 1996, and March 2004 Addendum. Such threadforms,
commonly known as interference threads, typically are tapered and advance in
the form
of a helix around the external circumference of the pin ends 18a-b and
internal
circumference of the collar ends 22a-b.
Each of the collar ends 22a-b further is formed as having an internal groove,
30a
and 30b, machined or otherwise formed therein the corresponding threads 24a-b
such as
intermediate the terminus, 31 a-b, of each of the collar ends 22a-b, and the
axial
centerline, 32, of the collar 14. Each of the grooves 30a-b may be generally
circular in
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extending circumferentially about the inner diameter of the corresponding
collar end 22a-
b, and may have a generally square, rectangular, or other cross-sectional
shape.
Grooves 30a-b are provided to receive a generally annular seal ring therein,
such
as the rings 34a and 34b shown in the assembly view of Fig. 1B. Such rings 34a-
h,
which may have a round, oval, square, rectangular, or other cross-section such
as a U-
shape, may be formed or a filled or unfilled elastomeric, plastic, or other
polymeric
material, which material may be filled or unfilled. For chemical resistance,
the polymeric
material forming the seal rings 34a-b often is specified to be a
fluoropolymer, which may
be a homopolymer or a fluoropolymer copolymer or blend, mixtures, alloy, or
other
combination. Representative fluoropolymers include fluorinated ethylene
polypropylene
(FEP) copolymer, perfluoroalkoxy {PFA) resin, polychlorotrifluoroethylene
(PCTFE)
copolymer, ethylene-chlorotrifluoroethylene (ECTFE) copolymer, ethylene-
tetrafluoroethylene (ETFE) terpolymer, polyvinylidene fluoride (PVDF),
polyvinylfluoride (PVF), and, particularly, polytetrafluoroethylene (PTFE). As
mentioned the material forming the seal rings 34a-b may be a homo or
copolymer, or a
combination thereof such as a blend of one or more homopolymers, one or more
copolymers, or one or more homapolymers and on.e or mare copolymers. Such
materials
each additionally may be admixed with other resins, which may be thermoplastic
or
thermosetting, such as to form an alloy, blend, mixture, or other combination,
or a
copolymer thereof. Preferred composition, whether filled or unfilled,
generally will
exhibit sufficient pliability for easy installation into the mounting groove,
good extrusion
resistance, and robust leak resistance to liquid or gas over a range, both low
and high, of
service pressures, and at low and high temperature extremes.
Although the material forming the seal rings 34a-b may be unfilled, such
material
typically is filled with up to about 30% by weight of glass particles for use
in liquid and
gas pressure service at temperatures up to about 350°F (175°C).
In accordance with the
precepts of the present invention, however, and for higher temperature
service, the
fluoropolymer or other polymeric material forming the rings 34a-b may be
filled with at
least about 25% by weight, and typically at Least about 50% by weight based on
the total
weight of the composition of one or more other particulate fillers such as a
metal or metal
alloy, which may be a steel oz a stainless or other corrosion resistant steel
(CRES) having
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CA 02526822 2005-11-14
an inherently high strength and temperature resistance, or a carbon fiber,
silica, ceramic,
or mica, or a combination thereof. Higher filler loading levels in general may
be used to
increase the service temperature of the material forming the rings 34a-b by
reducing the
volumetric expansion thereof at the higher operating temperatures.
For use in the field, the collar 14 is "bucked" onto one of the ends I8a-b of
the pin
12 to form the joint or connection 10 shown in Fig. 1B. Each connection 10
then is run,
one at a time, down the wellbore with the pin end 18b oriented downward and
the box
end 22b oriented upward, and is set in a vertical position. Once set, ~ the
second
connection 10 is positioned and, the pin 12 thereof is "stabbed" into the
collar 14 of the
first connection 10. The second connection 10 then is rotated with special
equipment
called tongs to make up the joint therebetween to what is known as the "power
tight"
position.
To assist in the visualization of the connection 10, reference may be had to
Figs.
2A and 2B wherein the end 18a of pin 12 is depicted somewhat schematically to
be in the
form of a truncated external cone which fits into the mating internal cane of
the end 22a
of the collar 14. However, the outer diametric extent of the external cone of
the pin 18a
is sized to be marginally larger than the inner diametric extent of the
internal cone of the
collar end 22a. In this regard, as depicted in Fig. 2A, upon a specific amount
of axial
advancement of the pin end 18a along axis 16 as the pin 12 is rotated into the
collar 14,
the respective threads 20a and 24a thereof enmesh completely at a point which
defines
what is known as the hand-tight position. Additional rotation and
corresponding axial
advancement of the pin end 18a along the remainder of the axial extent,
referenced at d,,
to the centerline 32 of the collar 14 requires the application of torque. The
torque
required to advance the pin end 18a along the extent dl increases steadily
until the final,
or power-tight position is reached, such as is shown in Fig. 2B, wherein the
pin end 18a
has been advanced over the axial extent referenced at d2 to a minimum power-
tight
condition, and which may be further advanced to a maximum power tight
condition as is
referenced in phantom at I8a'.
Thus, it may be appreciated that the assembly of the connection i0 forces the
larger conical section of the pin end I8a into the marginally smaller mating
conical
section of the collar end 22a. As a result, as these component parts are
mated, they must
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CA 02526822 2005-11-14
deflect in opposite directions from one another izzasmuch as the two parts
cannot occupy
the same space. That is, the collar end 22a is forced to expand radially
outward, with the
pin end 18a being forced to compress radially inwardly. The difference between
the
outward deflection of the collar end 22a and the inward deflection of the pin
end 18a is
the interference in the threads 20a and 24a.
In addition to radial deflection, the ends 18a and 22a of the pin 12 and
collar 14
are flexed longitudinally during assembly. Specifically, the collar end 22a
flexes
outwardly and axially elongates, while the pin end 18a flexes inwardly and
axially
Shortens.
Importantly, it also must be understood that the machining of the pin 12 and
collar
14 is not perfect inasmuch as the ends I8a and 22a thereof generally are not
perfectly
round and the wall thichnesses may not be consistent. Indeed, there may be
significant
variation in each of the components in the connection 10 as to roundness,
diameter and
wall thickness. The threads 20a and 24a and the groove 30a also may vary, such
as in
diameter and, in the case of the threads 20a and 24a, in thread height, lead
and taper.
Additional variability, moreover, may arise from the degree of the pin 12
travel into the
collar 14. Specified tolerances placed on certain of the nominal dimensions
help to limit
theses variations and ensure a more reliable connection under the expected
loading
conditions. These conditions can include axial tension and compression,
internal and
external pressures, and bending, or any combination thereof.
Now, considering the these geometric variations in the components of the
connection 10, it will be appreciated the void space in the groove 30a which
is to be filled
with the mass of the seal ring 34a also is subject to significant variation:
However, the
seal ring 34a can be optimally sized only if the volume of this space can be
accurately
calculated. Moreover, the seal ring 34a itself has specified dimensions and
tolerances
which introduces further variation into the calculus. The methodology
according to the
present invention involves mathematically modeling, such as by way of a finite
element
analysis (FEA), the geometry of the connection 10. Such model may include the
specified tolerance ranges far selected variables such as minimum and maximum
thread
interferences, makeup positions, and groove locations, as well as the groove
width and
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diameter. Minimum and maximum pin and collar diameters and wall thicknesses,
as well
as other variables, may be considered in the model.
Each model developed then may be processed using known computational
techniques and commercially available computer programs. Such programs are
used in
many fields to analyze mechanical parts and to relative movements between
parts.
Output from these programs typically includes deformed shapes, deflections,
stresses and
strains.
As to the present methodology, the modeling begins by developing and analyzing
a 2-dimensional, axisymmetric model which simulates makeup. Relative
deflections
between the components define an after-assembly deformed shape of the
connection 10
such as is depicted at 50 in Fig. 3A. The inset referenced at 52 shows in
enhanced detail
the void volume, 54, defined by the groove 30a and the threads 20a of the
mating pin 12
after assembly. The geometry of the void volume 54 prior to assembly is
referenced in
phantom at 54' for comparison.
The model 50 of Fig. 3A reappears at 60 on an enlarged (20X) scale, wherein
the
relative deflections of the pin i2 and collar 14 appear somewhat exaggerated
for a better
comparison with the original geometry thereof prior to assembly as shown in
phantom at
12' and 14'. From these Figs. 3A-B, one can appreciate how the deflection of
the pin 12
and collar 14 affects the area of the void volume 54.
With one or more of the models of Fig. 3A-B thus being developed, FEA then
may be are performed on these models, each of which may account for the above-
mentioned variations over the minimum and maximum extrema of the tolerance
range for
one or more of the nominal dimensions of the connection 1Ø A pictorial
representation
of the 2-dimensional void space between the seal ring groove 30a and the
mating section
of the pin thread 20a is depicted at 70 in Fig. 4. Such representation shoves
the relative
orientations of the groove and threads both prior to (30a' and 20a') and after
(30a and
20a) assembly. The change in the void space defined therebetween also may be
visualized by a comparison of the void spaces prior to (54) and after (54')
assembly.
Once the deformed shapes of the seal ring groove 30a and the mating section of
the pin threads 20a have been modeled, these 2-dimensional models then can be
used to
develop a 3-dimensional model of the void volume space by mathematically
revolving
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CA 02526822 2005-11-14
the 2-dimensional models radially about axis 16 as an axis of revolution to
generate a
corresponding solid axis of revolution for each of fhe deformed groove 30a and
the
mating section of the deformed threads 20a. As to the groove 30a, inasmuch as
the
groove is circular and radial, the conversion of the 2-dimensional model into
a 3
dimensional solid model is straightforward.
The conversion of the mating section of the deformed pin threads 20a is
complicated somewhat by the fact that the threads are helical and are machined
on the
taper of the pin end 18a. For example, API buttress threads have 5 threads per
inch on a
3/a inch per foot taper. One complete turn of an API buttress pin member into
a matingly
threaded collar therefore results in 0.200 inch of axial advancement. To
assist in the
visualization of how the helical threads 20a propagate relative to the annular
groove 30a
in defining the void volume 54, reference may be had to Fig. 5 wherein a
radial cross
section through a schematic of the connection 10 is depicted at 80. The insets
show the
progression in the geometry of the groove 54 at 45° intertrals as the
helical threads 20a
progress through the groove 30a with one turn or 360° rotation of the
pin 12.
The 2-dimensional deformed thread geometry thus is revolved helically about
axis
I6 to generate a true 3-dimensional solid model of the threadform geometry.
Such
geometry is depicted at 90 in Fig. 6, along with the solid model for the
groove 30a which
is depicted at 92.
. Within the connection 10, with groove 30a being aligned over the pin threads
20a,
the volumes of the solid models 90 and 92 of Fig. 6 can be subtracted to
determine the
actual deformed shape of the void volume 54 that must be filled by the seal
ring 34a.
(Fig. 1B). Such subtraction may be performed using the Boolean operations
typically
available in commercial computer modeling software.
The solid model of the void volume 54 which results after the subtraction is
depicted at 94 in Fig. 7, with Figs. 8-11 showing, respectively, the sections
95-98 thereof
taken at 90° intervals. Fig. 7 accurately depicts the void volume in
the assembled
connection, with such volume representing the gland that must he filled by the
"optimized" seal geometry. In Figs. 8-I l, the ends of the sections 95-98 are
labeled as
one of A-D corresponding to the axial cross-sectional geometry thereat the
lines A-D
referenced in Fig. 7.
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Using the above-describe methodology on standard 9-SJ8" OD casing
connections, it has been found that the void volume ranges from a minimum of
0.30
cubic inches to a maximum of 0.38 cubic inches in accounting for all material
combinations of minimum and maximum tolerance extrema in the connection. Armed
S with such knowledge of the minimum and maximum limits of the void volume,
the
designer can readily select, fvr example, optimal dimensions for the seal
rings 34a-b.
With reference to Fig. 12 wherein seal ring 34a reappears and, particularly,
to Fig. 13
wherein the ring 34a may be seen to have a generally rectangular cross-
section, such
dimensions can include the ring width, thickness, and outer diameter as
referenced,
respectively, at "w," "t," and "do" in Fig. 13. For mounting within the groove
30a, the
seal ring 34a may be sized as having an outer diameter do which is marginally
larger than
the inside diameter of the groove 30a such that, when mounted in the groove,
the ring
will be in a state of circumferential compression.
Overall, the designer may wish to limit the minimum seal overfill volume to a
minimum of about 2% and a maximum of about 25°l0. Such limits are
believed to be
practical in view of the tolerance constraints involved in machining and
assembling
connections such as used in the oil and gas industry. Of course, tighter
overfill tolerances
may be possible for connections used in other industries.
Ultimately, the seal ring dimensions and tolerances can be based on the more
precise void volume calculations determined using the design and sealing
methodology
herein involved. Advantageously, the methodology can be used to exercise more
control
over the ove~ll by specifying a more precise minimum and maximum range and, as
a
result, to reducing the potential for localized high stresses in the
connection which can
develop during assembly and otherwise in service with increases in
temperature.
Except as otherwise stated, the materials of construction for the componentry
of
the connection 10 may be considered conventional for the application involved,
and
generally may be selected for strength, corrosion or temperature resistance,
or other
physical or mechanical property, or otherwise for compatibility with the
service
environment, and/or the fluid being handled. Such fluid may be a liquid such
as water,
hydraulic oil, a crude oil or other hydrocarbon fuel or other petrochemical,
or a process
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CA 02526822 2005-11-14
stream. Alternatively, the fluid may be air, such as in a pneumatic
application, steam, or
another gas.
Although plastics, composites, and other materials may be used where the
application permits, the connection componentry in general may be machined,
cast,
molded, extruded, forged, or otherwise constructed of a metal, which may be
same or
different for each of the components, and which typically will be a steel but
which also
may be a copper, brass, stainless steel, titanium, or an aluminum, or an
alloy.
Thus, a unique methodology for modeling threaded tubular connections is
described such that seal rings therefor may be sized to avoid underfill and
excessive
overfill conditions, and thereby to improve the sealability of such
connections in either
normal or high service temperature applications. Such method, moreover, may be
used to
modify the design of such connections, and in the selection of materials and
material
formulations for the seal rings used in such connections. Such methodology may
be used
for industry standard connections or in the design of new connections or the
modification
of existing connections such as in the addition of a secondary or redundant
sealing
mechanism therefor. Indeed, use of the methodology herein may allow designers
to
adapt industry standard connections for use in new applications such as in
steam injection
and geothermal wells, and to thereby eliminate the need for proprietary or
other non-
standard connections, and the added costs thereof and for associated special
tooling and
other accessories, while simplifying logistics fox drilling, recovery, or
other project
involved.
As it is anticipated that certain changes may be made in the present invention
without departing from the precepts herein involved, it is intended that all
matter
contained in the foregoing description shall be interpreted in as illustrative
rather than in
a limiting sense. All references including any and all priority documents
cited herein are
expressly incorporated by reference.
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